Current commercially available 40 Gb/s 300 pin modules such as described in 300 pin MSA Group, Reference Document for “300 pin 40 Gb Transponder,” Public Document Edition 3, Jul. 19, 2002, found on the world wide web at 300pinmsa.org, use direct detection methods. These transponders are mostly based on four modulation formats: (a) 40 Gb/s NRZ (non-return-to-zero), (b) 40 Gb/s duobinary, (c) 40 Gb/s differential phase-shift-keying (DPSK), and (d) 2×20 Gb/s differential quadrature phase-shift-keying (DQPSK). 40 Gb/s NRZ has a severe dispersion-limited transmission distance of around 2˜10 km and is usually for short-distance “client-side” applications. 40 Gb/s duobinary modulation was used in the first-generation long-haul transmission systems, but has a severe limitation in terms of both poor chromatic dispersion (CD) and polarization mode dispersion (PMD) tolerance, 40 Gb/s DPSK and 2×20 Gb/s DQPSK both exhibit good optical signal-to-noise ratio (OSNR) performance. DPSK has limited CD and PMD tolerance, while DQPSK improves both CD and PMD tolerance by halving the symbol rate. However, DQPSK requires much more complicated structure than DPSK, duobinary, and NRZ, and therefore has a higher cost. 40 Gb/s DPSK and 2×20 Gb/s DQPSK require thermally-tuned phase demodulator. 40 Gb/s duobinary, 40 Gb/s DPSK, and 2×20 Gb/s DQPSK require thermally-tuned optical dispersion compensator. These thermally-tuned devices are all very slow, with a tuning time in the range of tens of seconds. As a result, none of these modulation formats are suitable for a ROADM (reconfigurable optical add-drop multiplexing)-based optical network, which needs to re-configure wavelengths dynamically and a fast traffic recovery time.
Another modulation technique is 2×20 Gb/s bandlimited-optical duobinary (BL-ODB). 2>20 Gb/s BL-ODB was proposed in J. Yu, et al., “Optical subchannels from a single lightwave source,” U.S. Patent Publication No. US2008/0063396 A1, published Mar. 13, 2008, and in L. Xu et al., “Spectral Efficient Transmission of 40 Gbps per Channel over 50 GHz Spaced DWDM Systems Using Optical Carrier Suppression, Separation and Optical Duobinary Modulation,” paper NTuC2, Optical Fiber Communications Conference, 2006. In these systems, both the optical modulator and photo-detector use complex 40 Gb/s components.
The concept of using structurally simpler and more common 10 Gb/s opto-electronic components for 40 Gb/s duobinary data was apparently first proposed in H. L. Lee et al., “Duobinary Optical Transmitter”, as disclosed in related U.S. Pat. No. 7,215,892 B2, issued May 8, 2007 and U.S. Pat. No. 7,224,907 B2, issued May 29, 2007.
From packaging perspective, although it is challenging to fit multiple opto-electronic components into a 40 Gb/s 300 pin MSA module, it is even more challenging to fit those opto-electronic components in a much smaller space offered by CFP MSA module as specified in CFP Draft 1.0, Mar. 23, 2009.
The C (Latin letter C for 100 or centum) form-factor pluggable (CFP) is from a multi-source agreement (MSA) for a standard common form-factor for high-speed transmission digital signals. The CFP supports 100 Gb/s and 40 Gb/s using 10 and 4 lanes in each direction (Rx and Tx), respectively, with 10 Gb/s in each lane.